For many applications, it is desirable to have a thermal energy source whose emitted radiant energy consists of photons that are confined either to a single wavelength or to a narrow band of wavelengths. For example, photovoltaic cells have unique spectral responses and can be optimized to efficiently convert electromagnetic radiation to electricity if the incident radiation on the cells is constrained to those frequencies or wavelengths most efficiently utilized by the photovoltaic cells. A common way of achieving a desired radiant energy spectrum from a thermal energy source is to select a material whose atomic or molecular properties constrain its emittance to selected frequencies or wavelengths. Examples of such materials are some gases, such as cesium, and certain rare earth oxides, such as yterbium oxide. Rare earth oxides are used for example to modify the emitted radiant energy from the burner element of an oil-burning lamp.
Difficulties with using gases and oxides are due to the natural constraints on their properties, i.e., they are usually constrained to a wavelength determined by their physical characteristics, which wavelengths cannot be adjusted or modified to match other desired frequencies. In the case of gases, it is difficult to efficiently couple radiant energy into desired light-emitting modes, and in the case of rare earth oxides, it is difficult to suppress out-of-band background emittance to achieve a sufficiently broad bandwidth profile in order to achieve high efficiency coupling of energy into the desired modes. Further, both media are transparent to broadband radiation from background sources such as mechanical mounting structures.